Heart valves, such as the mitral, tricuspid, aortic and pulmonary valves, are sometimes damaged by disease or by aging, resulting in problems with the proper functioning of the valve. Heart valve problems generally take one of two forms: stenosis, in which a valve does not open completely or the opening is too small, resulting in restricted blood flow; or insufficiency, in which blood leaks backward across a valve when it should be closed.
The pulmonary valve regulates blood flow between the right ventricle and the pulmonary artery, controlling blood flow between the heart and the lungs. Pulmonary valve stenosis is frequently due to a narrowing of the pulmonary valve or the pulmonary artery distal to the valve. This narrowing causes the right side of the heart to exert more pressure to provide sufficient flow to the lungs. Over time, the right ventricle enlarges, which leads to congestive heart failure (CHF). In severe cases, the CHF results in clinical symptoms including shortness of breath, fatigue, chest pain, fainting, heart murmur, and in babies, poor weight gain. Pulmonary valve stenosis most commonly results from a congenital defect, and is present at birth, but is also associated with rheumatic fever, endocarditis, and other conditions that cause damage to or scarring of the pulmonary valve. Valve replacement may be required in severe cases to restore cardiac function.
Previously, valve repair or replacement required open-heart surgery with its attendant risks, expense, and extended recovery time. Open-heart surgery also requires cardiopulmonary bypass with risk of thrombosis, stroke, and infarction. More recently, flexible valve prostheses and various delivery devices have been developed so that replacement valves can be implanted transvenously using minimally invasive techniques. As a consequence, replacement of the pulmonary valve has become a treatment option for pulmonary valve stenosis.
The most severe consequences of pulmonary valve stenosis occur in infants and young children when the condition results from a congenital defect. Frequently, the pulmonary valve must be replaced with a prosthetic valve when the child is young, usually less than five years of age. However, as the child grows, the valve can become too small to accommodate the blood flow to the lungs that is needed to meet the increasing energy demands of the growing child, and it may then need to be replaced with a larger valve. Alternatively, in a patient of any age, the implanted valve may fail to function properly due to calcium buildup and have to be replaced. In either case, repeated surgical or transvenous procedures are required.
To address the need for pulmonary valve replacement, various implantable pulmonary valve prostheses, delivery devices and surgical techniques have been developed and are presently in use. One such prosthesis is a bioprosthetic, valved conduit comprising a glutaraldehyde treated bovine jugular vein containing a natural, trileaflet venous valve, and sinus. A similar device is composed of a porcine aortic valve sutured into the center of a woven fabric conduit. A common conduit used in valve replacement procedures is a homograft, which is a vessel harvested from a cadaver. Valve replacement using either of these devices requires thoracotomy and cardiopulmonary bypass.
When the valve in the prostheses must be replaced, for the reasons described above or other reasons, an additional surgery is required. Because many patients undergo their first procedure at a very young age, they often undergo numerous procedures by the time they reach adulthood. These surgical replacement procedures are physically and emotionally taxing, and a number of patients choose to forgo further procedures after they are old enough to make their own medical decisions.
Recently, implantable stented valves have been developed that can be delivered transvenously using a catheter-based delivery system. These stented valves comprise a collapsible valve attached to the interior of a tubular frame or stent. The valve can be any of the valve prostheses described above, or it can be any other suitable valve. In the case of valves in harvested vessels, the vessel can be of sufficient length to extend beyond both sides of the valve such that it extends to both ends of the valve support stent.
The stented valves can also comprise a tubular portion or “stent graft” that can be attached to the interior or exterior of the stent to provide a generally tubular internal passage for the flow of blood when the leaflets are open. The graft can be separate from the valve and it can be made from any suitable biocompatible material including, but not limited to, fabric, a homograft, porcine vessels, bovine vessels, and equine vessels.
The stent portion of the device can be reduced in diameter, mounted on a catheter, and advanced through the circulatory system of the patient. The stent portion can be either self-expanding or balloon expandable. In either case, the stented valve can be positioned at the delivery site, where the stent portion is expanded against the wall of a previously implanted prostheses or a native vessel to hold the valve firmly in place.
One embodiment of a stented valve is disclosed in U.S. Pat. No. 5,957,949 titled “Percutaneous Placement Valve Stent” to Leonhardt, et al, the contents of which are incorporated herein by reference.
Over time, implanted prosthetic conduits and valves are frequently subject to calcification, causing the affected conduit or valve to lose flexibility, become misshapen, and fail to function effectively. Furthermore, because they are long term implants, synthetic conduits sometimes undergo longitudinal stretching or fibrotic ingrowth of the tissue surrounding the conduit. In either case, the conduit can become so distorted that blood flow is impeded or the valve is misaligned, allowing blood to flow around the periphery of the valve.
An additional drawback of using a stented valve is that the stents are often difficult to properly position within a conduit resulting in a misplaced valve. Additionally, stented valves may migrate along the conduit after implantation due to forces applied by the blood flow through the vessel.
It would be desirable, therefore, to provide an implantable pulmonary valve that can readily be replaced, and that would overcome the limitations and disadvantages inherent in the devices described above.